LoL this has been gone over in this thread. When I look at a fresh dosearch.php script right out of the .tar.gz file from the latest version release I have that error.
Oddly enough, someone sent me a dosearch.php script that they could not make work with the hack and I noticed they had the same error.
I have no idea why some have it (mostly me) and some dont.
So if your dosearch.php doesn't match the line that needs changing you probably shouldn't change it.